Bubble sizes in electrolyte and alcohol solutions in a turbulent stirred vessel

Bubble size distributions have been measured by a new video technique at 3 points near the wall in a vessel of 150 mm diameter air-sparged at similar to 1 vvm agitated by a Rushton turbine at an energy dissipation rate of similar to 1 W kg(-1). Water and solutions of electrolytes and alcohols were used. These solutes give surface tensions less than water (alcohols) and greater than water (electrolytes) and concentrations were chosen to produce solutions which, based on work in bubble columns and coalescence cells, can be considered partially-coalescing and non-coalescing. Regardless of surface tension, the bubble sizes in the non-coalescing solutions were approximately the same and much less than water, whilst those in the partially-coalescing case where the surface tension was approximately equal to that of water, gave intermediate sizes. Thus, the Weber number cannot correlate such results. On the other hand, the concept of bubble sizes being controlled by coalescence-inhibition after initial break-up works well. In all cases, bubbles as small as 40 mu m were found and even in water some 40% were below 300 mu m, the smallest size practicably measurable by a capillary technique. Surprisingly, the bubble size decreased with vessel height and possible reasons for this are discussed.